The present invention relates to a fuel cell, in particular an undivided fuel cell with electrolyte-filled electrodes.
Fuel cells are energy transformers which convert chemical energy into electrical energy. In a fuel cell, the principle of electrolysis is reversed.
Various types of fuel cells are known at the present time, and these differ from one another in, inter alia, the operating temperature. However, the structure of the cells is in principle the same in all types. They comprise, inter alia, two electrodes, namely an anode and a cathode, at which the reactions occur, and an electrolyte between the two electrodes. This has three functions. It establishes ionic contact, prevents electrical contact and also serves to separate the gases fed to the electrodes. The electrodes are generally supplied with gases which are reacted in a redox reaction. For example, the anode is supplied with hydrogen and the cathode is supplied with oxygen. To achieve this, the electrodes are provided with electrically conductive gas distribution devices. These are generally plates having a grid-like surface structure consisting of a system of fine channels. An exception is the direct methanol fuel cell in which the fuel is not gaseous but instead is an aqueous solution of methanol. The overall reaction in all fuel cells can be divided into an anodic substep and a cathodic substep. There are differences between the various types of cell in respect of the operating temperature, the electrolyte used and the possible fuel gases.
A fundamental distinction is made between low-temperature fuel cells and high-temperature systems. The low-temperature fuel cells generally have a very high electrical efficiency. However, the heat given off by them can be utilized only with difficulty because of the low temperature level. These fuel cells can therefore be utilized only for short-range heating and not for downstream energy transformation processes. Low-temperature fuel cells are therefore appropriate for mobile use and decentralized low-power applications. On the other hand, power generation stages can be installed downstream of the high-temperature systems in order to generate electric energy from the heat produced or to utilize the latter as process heat.
The present-day state of the art for fuel cells encompasses the following industrially relevant types:
AFC (alkaline fuel cells)
PEFC (polymer electrolyte fuel cell)
PAFC (phosphoric acid fuel cell)
MCFC (molten carbonate fuel cell)
SOFC (solid oxide fuel cell)
The polymer electrolyte fuel cell and the phosphoric acid fuel cell in particular are of great current interest both for stationary applications and for mobile uses and their broad commercialization is imminent. On the other hand, the other types are or were hitherto only operated in a few demonstration plants or for specific applications, e.g. in the spaceflight sector or for military purposes.
According to the present-day state of the art, all fuel cells have gas-permeable, porous electrodes, known as three-dimensional electrodes. These electrodes are referred to by the collective term gas diffusion electrodes (GDE). Through these electrodes, the respective reaction gases are conveyed close to the electrodes (cf. FIG. 1). The electrolyte present in all fuel cells ensures ionic charge transport in the fuel cell. It has the additional task of forming a gastight barrier between the two electrodes. In addition, the electrolyte guarantees and aids formation of a stable three-phase layer in which the electrolytic reaction can take place.
In alkaline fuel cells, the electrolyte can be a liquid. In the phosphoric acid fuel cell and the molten carbonate fuel cell, on the other hand, inorganic, inert supports form, together with the electrolyte, an ion-conductive and gastight matrix. In the solid oxide fuel cell, a high-temperature oxygen ion conductor generally serves as electrolyte and simultaneously as membrane. The polymer electrolyte fuel cell uses organic ion exchange membranes, in the industrially implemented cases perfluorinated cation exchange membranes, as electrolytes.
The structure of the electrodes and the type of electrolyte determine the three-phase boundary layer. Conversely, the structure of the boundary phase for each of these types of cell leads to specific demands on the gas diffusion electrode and the electrolyte. This leads to restrictions in terms of current density, temperature conditions and usability of support materials and of catalysts and auxiliaries. The sealing of the separate gas spaces at the anode and cathode leads to complicated construction and thus also to high cost and technical difficulty.
In all present-day fuel cells, the reaction gases are supplied to the electrochemically active zone from the reverse side of the electrode, i.e. in each case the side facing away from the counterelectrode, by means of a gas distributor system. Under load, both gas transport and ion migration occur perpendicular to the given electrode geometry, with the ions migrating between the electrodes and the gases migrating to the electrodes on the reverse side. In overall terms, therefore, gas transport and ion transport proceed in parallel (cf. FIG. 1). This has the consequence that good gas transport to the boundary between the two electrodes would lead to mixing of the reaction gases at this boundary if this were not prevented by specifically provided separation media or, as in some AFCs, by defined flow of electrolyte over the electrodes. Mixing of the reaction gases has to be avoided for safety reasons. In addition, gas going over to the other electrode would lead to mixed potential formation at the respective electrode. This would result in a significant reduction in power. However, the difficulty of providing suitable transport and separation measures substantially restricts the cost-effectiveness and the efficiency of today""s fuel cells. In addition, this working principle of present-day fuel cells makes the water balance and the heat management of the cell more difficult. In the case of a PEFC, for example, the water which forms has to be removed from the cell so that the gas diffusion electrodes do not xe2x80x9cdrownxe2x80x9d while, on the other hand, the system has to be kept sufficiently moist to ensure that the membranes remain conductive. Furthermore, owing to the materials which withstand little thermal stress, in particular the ion exchange membrane, heat removal is likewise an important criterion for the long-term efficiency of the cells.
It is an object of the present invention to provide a fuel cell in which the disadvantages inherent in the above-described working principle of today""s fuel cells are avoided.
We have found that this object is achieved by a fuel cell which comprises at least the following elements:
a) two electrodes which are each provided with at least one gas duct for a reaction gas,
b) a liquid electrolyte,
where the respective gas ducts of the electrodes have at least one inlet and run perpendicular to the migration direction of the ions under load prescribed by the arrangement of the electrodes.
For the purposes of the present invention, xe2x80x9cperpendicularxe2x80x9d means an angle between the gas duct and the migration direction of the ions which is in a range of 90xc2x0xc2x145xc2x0, preferably 90xc2x0xc2x120xc2x0 and particularly preferably 90xc2x0xc2x110xc2x0.
As a result of the reaction gases being conveyed perpendicular to the ions according to the present invention, mixing of the reaction gases at the boundary between the two electrodes is avoided without appropriate separation media or, as in the case of AFCs, appropriate flow of electrolyte over the electrodes, having to be provided. This considerably improves the economics and the efficiency of the fuel cells of the present invention compared to previously known fuel cells. Thus, there is no longer a need for a membrane to keep the reaction gases apart.
In a further preferred embodiment of the invention, the two electrodes are electrolyte-filled electrodes or electrodes which can be filled with electrolyte. The respective electrochemically active zones are thus not restricted to the respective surfaces of the two electrodes or corresponding regions between the two electrodes, but can occupy a considerably larger space, depending on the configuration of the gas ducts within the respective electrodes. The fuel cells of the present invention are thus significantly more economical than previous cells which have only very limited, in spatial terms, electrochemically active zones. Furthermore, the use of gas diffusion electrodes, i.e. of porous, gas-permeable bodies which bring about gas transport to the electrochemically active layer, are no longer necessary. The gas diffusion, electrodes uses hitherto are complex multicomponent sisters with a set hydrophilicity or hydrophobicity, porosity, catalyst loading or conductivity. They are very expensive and sometimes also chemically unstable. The omission of the membranes which has now become possible also reduces the costs considerably. In addition, the configuration according to the present invention also eliminates voltage losses which hitherto occurred both in the gas diffusion electrodes and in the indispensable membranes or other separation media. As a result, the energy yield and thus the total efficiency of the fuel cell of the present invention is increased compared to the prior art.
In a preferred embodiment of the invention, the fuel cell is thus an undivided cell since, when using the principle of the present invention, separation of the electrode spaces is no longer absolutely necessary and the reaction gases are no longer at risk of mixing with one another in an uncontrolled fashion. A divided cell is therefore no longer necessary for safety reasons.
In another embodiment of the invention, the fuel cell has a distributor system for introduction of in each case at least one reaction gas into the gas duct or ducts of the two electrodes. Preference is given here to using structures which are known in principle. Particular preference is given to using micromixers [V. Hessel, W. Ehrfeld, K. Golbig, V. Haverkamp, H. Lxc3x6we, T. Richter, Proceedings of the 2nd International Conference on Microreaction Technology, New Orleans, 1998]. The distributor system is, according to the present invention, designed so that a uniform, fine stream of gas bubbles rises in the electrodes. This increases the current density of the system. As reaction gases, preference is given to using oxygen and hydrogen. However, the use of methanol or methane is also conceivable here. In another preferred embodiment of the invention, the reaction gases are diluted with at least one suitable inert gas. Preference is given here to using nitrogen. The use of CO2 as inert gas is also conceivable. The reaction gases, preferably diluted with at least one inert gas, are preferably fed from below via the distributor system of the present invention into the fuel cell chamber and supplied to the corresponding gas ducts within the respective electrodes so that they are conveyed perpendicular to the ions. This means, for example, that the reaction gases flow parallel to the gap between the two electrodes, i.e. the gap between anode and cathode.
In a preferred embodiment of the invention, the two or more electrodes can be filled with at least one electrolyte.
In the present invention, the electrolyte used is a liquid which conducts ions. Preference is given here to an aqueous alkali metal hydroxide solution or an aqueous mineral acid solution, for example sulfuric acid, phosphoric acid or a hydrohalic acid. In another preferred embodiment of the invention, an organic electrolyte is used. Preference is given here to tetraalkylammonium hydroxides or tetraalkylammonium salts, sulfonic acids or phosphonic acids. However, all other suitable electrolytes can also be used.
In a preferred embodiment of the invention, water is used as solvent. In another preferred embodiment, water-miscible solvents such as carboxylic acids, alcohols, carboxamides and/or substituted ureas are used. In a further preferred embodiment of the invention, liquid and/or molten salts, for example tetraalkylammonium salts, 1,3-dialkylimidazolium salts and/or tetrachloroaluminates, e.g. NaAlCl4, are employed.
For the purposes of the present invention, the respective gas ducts of the electrodes can run either parallel or antiparallel, i.e. in countercurrent, to one another.
In a further preferred embodiment of the invention, the two electrodes are not arranged vertically, but have any angle of inclination and can even be arranged horizontally. The fuel cell can thus be configured in a manner appropriate to the demands made of it. It can be made very compact and thus space-saving.
The gap between the two or more electrodes preferably has a planar geometry. However, in another preferred embodiment of the invention, the gap can have a geometry other than planar, e.g. the two or more electrodes are arranged relative to one another so as to form an annular gap.
The electrodes have an inlet for the respective gas ducts and preferably also at least one corresponding outlet.
In a further, preferred embodiment of the invention, the width of the electrodes differs going from the inlet of the respective gas ducts to the outlet of the corresponding gas ducts. The inlet is preferably wider than the corresponding outlet, since a large amount of gas passes through the inlet while a smaller amount of gas passes through the outlet as a result of consumption. Thus, for example, the electrode/gap assembly forms a frustrum of a pyramid, a frustrum of a cone or has, for example, the cross section of a trapezoid. However, other geometric arrangements are also conceivable.
In a further, preferred embodiment of the invention, at least one of the two or more electrodes comprises a plurality of individual components. In one preferred embodiment of the invention, it comprises a plurality of parallel plates joined to form a lamellar structure. In the resulting intermediate spaces, the gas is conveyed parallel to the outer edges from one end face to the opposite end face. The distributor system supplies gas bubbles and takes up residual gases again. Within the circulated electrolyte system, the gas bubbles remain in the electrode.
Such electrodes are known from DE 41 20 679. There, these are referred to as capillary gap electrodes. In contrast to the present invention, however, the gas is supplied to the system from the reverse side of the electrode so that ion and gas transport occur in a parallel direction, as a result of which the disadvantages mentioned at the outset also occur when using these capillary gap electrodes. In addition, DE 41 19 836 expressly states that gas transport in the xe2x80x9csurface regionxe2x80x9d is easier than in the interior of the respective electrode and not vice versa. For this reason, a divided fuel cell is also absolutely necessary here. A preferred embodiment of the present invention is distinguished from these cells in that the electrolyte space between two electrode lamellae is structured so that gas transport takes place perpendicular to ion transport.
In a preferred embodiment of the invention, the surface of the electrodes is structured so as to form channels which run in the same direction as the gas duct or ducts. The profile of the channels can be semicircular, rectangular, triangular or of any other shape. The channels are preferably formed in the solid electrode material by milling, etching and/or other techniques. In another preferred embodiment of the invention, the channels are generated by appropriate corrugation of metal sheets or meshes. In a further preferred embodiment of the invention, the channels are produced by electroplating techniques in which the desired metal and/or alloy is deposited on a substrate which is appropriately masked by means of a template. Furthermore, other structures such as slotted tubes, wire bundles or drilled porous metal bodies or a suitable combined arrangement of these structures are also conceivable.
A particular advantage of the fuel cell of the present invention is that it provides new degrees of freedom in terms of the electrocatalytically active electrode surface. Since the individual subelectrodes, i.e. the individual lamellae, can be treated in any way prior to assembly to form the overall electrode, they can be activated by means of catalysts in an appropriate manner. This is achieved by coating with electrocatalytically active materials, for example with noble metals such as platinum, palladium, silver, ruthenium or iridium or combinations of these. This can be carried out, in particular, by electrolytic coating or electroless deposition of metal. A particularly advantageous procedure is described, for example, in DE 199 15 681.6.
In a further, preferred embodiment of the invention, the fuel cell additionally has at least one spacer which is arranged in such a way that it interacts functionally with the electrodes. Within the fuel cell, the electrodes are combined with spacers, so that electrodes made up of a plurality of individual elements are used. The electrodes particularly preferably comprise plates arranged in parallel which are joined to form a lamellar structure. The individual electrically conductive lamellae are kept apart by spacers so that passage of gas within the electrodes is ensured. According to the present invention, such a spacer has at least the following constituents:
a) a spacing-determining frame,
b) a window,
c) a gas barrier.
The spacing-determining frame defines and ensures the spacing of the individual lamellae. The window ensures unhindered gas and electrolyte flow between the lamellae of an electrode. The gas barrier consists of a web between the windows of anode and cathode and prevents gas bubbles from passing into the region of the respective counterelectrode. The web can be configured in various ways. For example, it can have a corrugated or folded structure. In a particularly preferred embodiment of the invention, the web is provided with further spacing elements, by which means the functional structure of the entire electrode/spacer unit is additionally stabilized.